Contact lenses generally used at present are broadly classified into a hard contact lens and a soft contact lens.
Concerning the hard contact lens, several types of lenses containing a novel material have been developed in recent years. However, all of such hard contact lenses have a defect in that the fitting sense is poor due to hardness of the material thereof.
On the other hand, as a soft contact lens, a lens produced from a hydrous material containing polyhydroxyethyl methacrylate as a main component is widely used. Since, however, such a soft contact lens has insufficient oxygen permeability and insufficiently supplies the cornea with oxygen, a considerable burden is eventually imposed on the cornea due to oxygen deficiency when the soft contact lens is fitted on for a long period of time.
In order to increase the amount of the oxygen supply to the cornea, therefore, there is employed a means of decreasing the thickness of a contact lens, or a means of increasing the water content of a lens material by using N-vinylpyrrolidone, etc., as a main component. However, these means cause the following problems. When the lens thickness is decreased, the lens form stability decreases, and it becomes difficult to maintain the optical lens function. There is hence a limit to decreasing the lens thickness. When the water content in a lens material is increased, the resultant lens is liable to be soiled, or is liable to be broken due to a decrease in mechanical strength.
In order to overcome the above problems, there has been proposed a hydrous soft contact lens which is produced from a material having oxygen permeability in itself (JP-A-54-29660). The contact lens disclosed in the JP-A-54-29660 is formed essentially of a copolymer of 2-hydroxyethyl methacrylate (to be referred to as HEMA hereinafter) and perfluoroalkylethyl methacrylate (to be referred to as RfMA hereinafter), and this contact lens has attained an oxygen permeability of 500 to 2,708 centibarrels.
Meanwhile, it is well known that with an increase in oxygen supply to the cornea when a contact lens is fitted on, i.e. with an increase in oxygen permeation coefficient of a contact lens, the burden on the cornea is alleviated with regard to metabolism.
In some recent reports on the relationship between an oxygen demand of the cornea when contact lens is fitted on and the oxygen permeation coefficient of a contact lens, the possibility to fit it on extendedly or continuously is discussed. However, no theory has been established on the lowest value of the oxygen permeation coefficient of a contact lens which enables continuous fitting. It is clinically reported that some contact lenses having an oxygen permeation coefficient of 16.times.10.sup.-11 to 21.times.10.sup.-11 [cm.sup.3 (STP) cm/(cm.sup.2 .multidot.sec.multidot.mmHg)] have been tested on continuous fitting. However, there is also another opinion that the above oxygen permeation coefficient values are too low to fit on a contact lens continuously. Therefore, if it is intended to prevent a hindrance to the cornea metabolism when a contact lens is fitted on, it is considered that a contact lens having an oxygen permeation coefficient of not less than 30.times.10.sup.-11 [cm.sup.3 (STP) cm/(cm.sup.2 .multidot.sec.multidot.mmHg)] greatly reduces a burden which a contact lens would put on the cornea when fitted on.
However, the contact lens disclosed in the above JP-A-54-29660 has an oxygen permeation coefficient of 5.times.10.sup.-11 to 27.times.10.sup.-11 [cm.sup.3 (STP) cm/cm.sup.2 .multidot.sec.multidot.mmHg)] after the conversion of the data disclosed therein, and it hence cannot be said that a sufficient oxygen permeation coefficient for reducing the burden on the cornea has been attained. That is, the RfMA used as an essential component for the copolymer forming the above contact lens essentially has high hydrophobic nature, and therefore, in order to improve the fitting sense of the lens, it might be attempted to increase the water content of the lens by decreasing the RfMA content and increasing the HEMA content. In this case, however, it is impossible to prevent a decrease in oxygen permeability due to an increase in the HEMA content, and the burden on the cornea increases due to oxygen deficiency. On the other hand, when the oxygen permeability is improved by decreasing the HEMA content and increasing the RfMA content, the resultant contact lens suffers a decrease in its hydrous nature and tends to be fragile.
This invention has been made in order to overcome the above problems, and provides a novel hydrous contact lens. It is an object of this invention to provide a contact lens having hydrous nature and sufficient oxygen permeability to reduce a burden which the contact lens might impose on the cornea when fitted on.